I recently finished playing Professor Layton and the Curious Village. Yes, the first Layton game -- I haven't had my DS for very long and have a lot of catching up to do.
I've been thinking about what makes a puzzle satisfying. Too easy and it's not really worthy of being called a puzzle. But too hard and I'll soon check a walkthrough rather than fruitlessly bashing my head against it.
Everyone finds different things challenging, though. For example, my partner has me well and truly beaten on number problems, while I have an easier time with spatial puzzles. Puzzle difficulty is quite individual. That said, I've yet to meet someone who didn't consider that map puzzle in Riven too damn hard. If you have played it you'll probably remember the one I mean.